1. Field of the Invention
The present invention relates generally to combined type transformers and buck-boost circuits using such a combined type transformer, and more particularly to an improved combined type transformer having both of inductor and transformer functions and an improved buck-boost circuit using the improved combined type transformer.
2. Description of Background Art
Heretofore, there have been proposed various voltage boosting circuits (for example, JP 2003-111390 A and JP 2003-216255 A). FIG. 22 hereof shows a fundamental circuit structure of the conventional voltage boosting circuit 100 disclosed in one of the above-identified publications.
As shown in FIG. 22, the voltage boosting circuit 100 includes an input-side smoothing capacitor 102, a coil (inductor) 101, a switching element 103, a diode 104, and an output-side smoothing capacitor 105. The input-side smoothing capacitor 102 is connected between a negative-pole terminal 106 and an input terminal 107 that is a positive-pole terminal, and the output-side smoothing capacitor 105 is connected between a negative-pole terminal 106 and an output terminal 108 that is a positive-pole terminal. DC reference line 110 is provided between the two negative-pole terminals 106. The switching element 103 is connected between a node 109, interconnecting the coil 101 and a node 104, and the DC reference line 110. The switching element 103 is a transistor, such as an Insulated-Gate Bipolar Transistor (IGBT), which has characteristics of both a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and a bipolar transistor. Gate signal SG111 is supplied by a not-shown control unit to a gate-source terminal of the switching element 103 so that ON/OFF control of the switching element 103 is performed on the basis of the supplied gate signal SG111.
Predetermined input voltage is applied between the input terminal 107 and the negative-pole terminal 106. Once the switching element 103 is turned on, an electric current flows through a loop, constituted by the coil 101, switching element 103, node b and node a, on the basis an electric charge stored in the input-side smoothing capacitor 102. During that time, the coil 101 is energized so that magnetic energy is stored in the coil 101. Then, once the switching element 103 is turned off, the magnetic energy stored in the coil 101 is discharged to the output-side smoothing capacitor 105. As a consequence, an output voltage greater than the input voltage applied between the input terminal 107 and the negative-pole terminal 106 is produced between the output terminal 108 and the negative-pole terminal 106. Intensity of the output voltage depends on the input voltage, switching duty, etc.
With the aforementioned voltage boosting circuit 100, where the voltage is boosted by temporarily storing the magnetic energy in the single coil 101, the coil 101 has to be extremely increased in size and weight, in order to store therein sufficient magnetic energy without causing magnetic saturation. Further, if an attempt is made to increase a voltage boost ratio in the voltage boosting circuit 100, undesired magnetic saturation of the magnetic core and lowering of the voltage boost ratio may occur.
In view of the foregoing, the assignee of the instant application has proposed a buck-boost DC/DC converter which permits reduction in size and weight of the coil (inductor) while reliably preventing magnetic saturation of the coil 101 and which can continuously vary the voltage-booting ratio (see JP 2006-149054 A).
FIG. 23 is a circuit diagram showing the DC/DC converter 200 disclosed in JP 2006-149054 A. The DC/DC converter 200 includes an inductor (coil) L0, a transformer T1, a core 221 and diodes 22, 23, 24, 25. FIGS. 24A and 24B and FIGS. 25A and 25B show a conventionally-known inductor and transformer applicable to the DC/DC converter 200 of FIG. 23.
FIG. 24A is a perspective view of the inductor 230 applicable to the DC/DC converter 200, and FIG. 24B is a schematic plan view of the inductor 230. Further, FIG. 25A is a perspective view of the transformer 240 applicable to the DC/DC converter 200, and FIG. 25B is a schematic plan view of the transformer 240.
As shown in FIGS. 24A and 24B, the inductor 230 includes a core section 232, an insulator 234, and a winding 236 having terminals 236a and 236b. The insulator 234 insulates the core 232 and winding 236 from each other. The winding 236 is wound around a center core 238 of the core section 232.
As shown in FIGS. 25A and 25B, the transformer 240 includes a core section 242, and windings 244 and 246 having respective terminals 244a, 244b and 246a, 246b. The windings 244 and 246 are wound around a center core 248 of the core section 242 and insulated from each other via an insulting sheet or the like.
The terminal 236a shown in FIG. 24A is connected to an input terminal TA1 shown in FIG. 23, and the terminals 244a and 2346a shown in FIG. 25A are both connected to the terminal 236a shown in FIG. 24A. Further, the terminal 244b shown in FIG. 25A is connected to a switching element SW1 shown in FIG. 23, and the terminal 246b shown in FIG. 25A is connected to a switching element SW3 shown in FIG. 23.
In the case where the inductor and transformer, constituting the DC/DC converter, are provided as separate components as set forth above, it would be difficult to reduce the size and weight of the DC/DC converter. Further, in the case where the conventionally-known inductor 230 and transformer 240 as shown in FIGS. 24A and 24B and 25A and 25B are employed, magnetic fluxes produced by electric currents flowing through the coils would undesirably disperse because projecting portions of the coils (windings) (which project outwardly from the core sections in the plan views of FIGS. 24B and 25B) and connecting portions of the coils (windings) between the terminal 236b of FIGS. 24A and 24B and the terminals 244b and 246b of FIGS. 25A and 25B have great cubic volumes. This is because the magnetic fluxes passing through the cores in response to the electric currents flowing through the coils are relatively reduced if the projecting portions and connecting portions of the coils are great in cubic volume. As a consequence, the (mutual) inductance would undesirably stay at low values as compared to the cubic volumes of the coils, which also makes it difficult to reduce the size and weight of the DC/DC converter. Also, the large connecting portions between the inductor and the transformer would undesirably lower the power conversion efficiency due to a large conduction loss.